Advances in semiconductor device technologies have included the development of a number of complex, intricate structures. Examples of such structures include cross-hair cells, which may be used in dynamic random access memory (DRAM) devices, the epi-diodes of phase-change random access memory (PCRAM) devices, and pseudo silicon-on-insulator (PSOI) structures. Fabrication of these and other structures typically requires the use of two or more masks, through which material is removed to define the desired structure or structures.
Once material has been removed through a first mask, the upper, or “active,” surface of the substrate typically includes a number of recesses. As a consequence of the presence of these recesses, one of two undesirable results will occur when conventional conformal mask fabrication processes are used to form a subsequent mask: (1) the recesses must be filled with a sacrificial material before the layer of mask material is applied and the subsequent mask is formed therefrom; or (2) a layer of mask material, such as a photoresist, that will be used to define the subsequent mask will conform to the topography of the semiconductor device structure. When the recesses are not filled before mask material is applied, mask material enters the recesses, resulting in a layer of mask material with a nonplanar surface. The degree of nonplanarity corresponds to the size and/or density of recesses, with the nonplanarity being more pronounced when a substrate includes larger and/or more densely arranged recesses.
When a conformal mask is formed, different regions of the layer of mask material are located at different elevations. When a layer of photoresist is applied to the layer of mask material, it too will have different regions that are located at different elevations. Unfortunately, the tools that are used to expose the photoresist have a narrow focal plane, which may not coincide with all areas of the photoresist, causing some areas to be exposed to a blurred radiation pattern. When the exposing radiation is blurred, the precision with which mask features (e.g., solid edges, apertures, etc.) may be formed through the photoresist is reduced, as is the precision with which these features may be transferred into the mask material from which the subsequent mask will be defined.
When the recesses are filled before the mask material is applied, the semiconductor device structure must be subjected to a number of additional processes, including material deposition, planarization, and material removal processes, all of which undesirably add to the overall cost of fabrication, as well as increase the probability that the resulting semiconductor devices will be damaged.